The present invention relates to an apparatus for vehicle hydraulic brake systems, particularly dual hydraulic brake systems having disc-type brakes on the front wheels and drum-type brakes on the rear wheels.
It is known that the proportion of the vehicle's weight that is borne by the front and rear wheels of a vehicle changes as the vehicle is braked. In particular, during the braking operation the weight borne by the front wheels increases while the weight borne by the rear wheels decreases. The change in weight is dependent upon the magnitude of the deceleration of the vehicle and also the weight of the vehicle and the position of the load in it. To compensate for this weight shift, various proportioning devices or valves are known today which are responsive to master cylinder outlet pressures and which serve to limit the brake pressure applied to the rear wheels upon attainment of a predetermined master cylinder outlet pressure.
In vehicular brake systems having dual hydraulic brake systems with disc-type brakes on the front wheels and drum-type brakes on the rear wheels, it is also necessary to have the braking forces and the torques be applied approximately simultaneously to the front and rear wheels. This prevents a dangerous skidding condition. Further, in such vehicular brake systems, the front and rear brakes are designed to reach a point of lock-up at a predetermined pressure ratio. Since disc-type and drum-type brakes have different feed characteristics, the fluid pressure at which the rear brakes will lock up under prolonged braking conditions is considerably greater than the fluid pressure at which the front wheel disc brakes will lock up under the same braking conditions. Thus, it is desirable that a decreasing offset or pressure differential between the front and rear brake pressures be established to achieve equalization of the fluid pressure delivered to the front and rear brakes at a predetermined rate.
Further, with the use of dual hydraulic brake systems in vehicles having radical weight shifts in the laden and unladen conditions, such as light trucks, recreational vehicles, small buses, front wheel drive cars, and other similar vehicles, it is necessary to match the braking pressures in the front and rear wheels to the available road adhesion in both conditions.
It is a general object of the present invention to provide a new, improved proportioning device which is adapted to be inserted between the master cylinder and a rear brake cylinder of a vehicle braking system for controlling the pressure applied to the rear brakes above a first predetermined master cylinder outlet pressure and also having an additional proportioning device incorporated therein for controlling the pressure delivered to the rear brakes above a second predetermined brake fluid inlet pressure, in order to accommodate for differential fade characteristics between the front and rear brakes of a vehicle.
It is another object of the present invention to provide a proportioning device which is particularly suitable for vehicles with radical weight shifts in the laden and unladen conditions in order to make maximum use of available road adhesion in both conditions. It is a further object to provide a device meeting the above-stated conditions which can be substituted in conventional differential area proportioning valves without any additional body machining or other changes in the housing or other parts of the braking system.
It is still another object of the present invention to provide a device for dual master cylinder braking systems which prevents the front disc-type brakes from being energized until the rear drum-type brakes are functioning, thereafter meters the pressure applied to the front brakes and limits the pressure applied to the rear brakes, and also includes means for indicating to the vehicle operator of a failure in either the front or rear brake system.
Other objects, features and advantages of the present invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings.